Method of making cellular polyethylene

Claim 1

IN A PROCESS FOR MAKING POLYETHYLENE FORM WHEREIN A POLYMERIC COMPOSITION COMPRISING ESSENTIALLY POLYETHYLENE HAVING A NORMALLY GASEOUS AGENT INTIMATELY INCORPORATED THEREWITH UNDER PRESSURE AT TEMPERATURES ABOVE THE MELTING POINT OF THE POLYMER IS EXPANDED BY SUDDENLY REDUCING THE PRESSURE SUFFICIENTLY TO CAUSE EXPANSION OF THE COMPOSITION WITH RESULTANT FORMATION OF A FOAM, THE STEPS WHICH CONSIST IN SUBJECTING POLYETHYLENE TO AN AMOUNT OF JOINING RADIATION CORRESPONDING TO A TOTAL DOSE OF FROM 2 TO 20 MEGA-R.E.P.''S SUFFICIENT TO CAUSE THE POLYETHYLENE TO BE INSOLUBEL IN BOILING DECALIN AND INSUFFICIENT TO PREVENT APPRECIABLE FLOW OF THE POLYETHYLENE AT TEMPERATURES BETWEEN 100* AND 200*C., PLACING THE IRRADIATED POLYETHYLENE UNDER PRESSURE OF A NORMALLY GASEOUS AGENT SELECTED FROM THE GROUP CONSISTING OF TETRAMETHYLMETHANE, MONOCHLORODIFLUOROMETHANE, DICHLORODIFLUOROMETHANE AND DICHLOROTETRAFLUORETHANE, AT TEMPERATURES ABOVE TRHE CRYSTALLINE MELTING POINT OF THE POLYETHYLENE AND WITHOUT MECHANICAL WORKING OF THE HEAT-PLASTIFIED POLYMER AND SUDDENLY RELEASING THE PRESSURE.